It is known that apparent picture sharpness may be enhanced by modulation of the scanning beam velocity in accordance with a derivative of a video display signal. The derivative signal, or SVM signal, may be derived from a luminance component of the video display signal and is employed to produce scanning beam velocity variations. Slowing of the electron beam velocity results in brightening of the display, acceleration of the velocity results in display darkening. Thus, horizontal rate edges may be visually enhanced by a variation of the display intensity about an edge transition. This method of sharpness enhancement provides various advantages over that provided by video frequency peaking, for example, blooming of peaked high luminance picture elements is avoided, and additionally, video noise occurring within the bandwidth of a video peaker is not enhanced.
Scanning beam velocity may be modulated by an SVM coil which generates a supplementary deflection field. The SVM field, in conjunction with the main deflection field, produces electron beam acceleration or deceleration responsive to the polarity of current in the SVM coil. The amount of beam acceleration or deceleration is proportional to the magnitude of the SVM current. The deflection sensitivity of a typical SVM coil, may for example, be in the range where 1 ampere produces between 1 to 3 millimeters of beam deflection at center screen.
Since the SVM signal generally represents high frequency video content, it can be appreciated that the SVM coil current has a magnitude and frequency spectrum which is readily coupled to produce unwanted, extraneous crosstalk components. Such crosstalk components may result from coupling via the power supply and or return circuit. Hence it is advantageous to arrange that SVM coil current be generated and circulate without significant ground or power supply conduction.